Modified start sequence of a gas turbine engine

ABSTRACT

A system for starting a gas turbine engine of an aircraft is provided. The system includes a pneumatic starter motor, a discrete starter valve switchable between an on-state and an off-state, and a controller operable to perform a starting sequence for the gas turbine engine. The starting sequence includes alternating on and off commands to an electromechanical device coupled to the discrete starter valve to achieve a partially open position of the discrete starter valve to control a flow from a starter air supply to the pneumatic starter motor to drive rotation of a starting spool of the gas turbine engine below an engine idle speed.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly to an apparatus, system and method for modifying a start sequence of the gas turbine engine.

Gas turbine engines are used in numerous applications, one of which is for providing thrust to an airplane. When the gas turbine engine of an airplane has been shut off for example, after an airplane has landed at an airport, the engine is hot and due to heat rise, the upper portions of the engine will be hotter than lower portions of the engine. When this occurs thermal expansion may cause deflection of components of the engine which may result in a “bowed rotor” condition. If a gas turbine engine is in such a “bowed rotor” condition it is undesirable to restart or start the engine.

Accordingly, it is desirable to provide a method and/or apparatus for mitigating a “bowed rotor” condition.

BRIEF DESCRIPTION

In an embodiment, a system for starting a gas turbine engine of an aircraft is provided. The system includes a pneumatic starter motor, a discrete starter valve switchable between an on-state and an off-state, and a controller operable to perform a starting sequence for the gas turbine engine. The starting sequence includes rapidly alternating on and off commands to an electromechanical device coupled to the slower moving, discrete starter valve to achieve a partially open position of the discrete starter valve to control a flow from a starter air supply to the pneumatic starter motor to drive rotation of a starting spool of the gas turbine engine to a dry motoring speed below a shaft resonance speed which is also below an engine idle speed.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the electromechanical device has a cycle time defined between an off-command to an on-command to the off-command that is at least half of a movement time for the discrete starter valve to transition from fully closed to fully open.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the electromechanical device is a solenoid that positions the discrete starter valve based on intermittently supplied electric power.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the electromechanical device is an electric valve controlling muscle air to adjust the position of the discrete starter valve.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller modulates the on and off commands to the electromechanical device to further open the discrete starter valve and increase a rotational speed of the starting spool.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include an electronic engine control system that includes a memory for recording a current heat state of the gas turbine engine at shutdown and for recording a shutdown time of the gas turbine engine. The electronic engine control system further includes a risk model for determining a time period (t_(motoring)) for motoring the gas turbine engine at about a predetermined speed range N_(target+/−)N where the predetermined speed is less than a speed used to start the gas turbine engine and where t_(motoring) is a function of the heat state recorded at engine shutdown and an elapsed time of an engine start request relative to the previous shutdown time.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the time period (t_(motoring)) is calculated automatically during a start of the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller modulates a duty cycle of the discrete starter valve via pulse width modulation.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the predetermined speed N_(target) is within a predetermined speed range N_(targetMin) to N_(targetMax) that is used regardless of the calculated time period t_(motoring).

According to an embodiment, a gas turbine engine and a system for controlling a start sequence of the gas turbine engine includes an electronic engine control system, a thermal model, memory, a model for determining a time period (t_(motoring)), and a controller. The thermal model is resident upon the electronic engine control system and configured to synthesize a heat state of the gas turbine engine. The memory is for recording the current heat state of the gas turbine engine at shutdown and for recording a shutdown time of the gas turbine engine. The model for determining the time period is for motoring the gas turbine engine at a predetermined speed N_(target) wherein the predetermined speed is less than a speed used to start the gas turbine engine and wherein t_(motoring) is a function of the heat state recorded at engine shutdown and an elapsed time of an engine start request relative to a previous shutdown time. The controller is for modulating a starter valve of a starter of the gas turbine engine in order to maintain the gas turbine engine within a predetermined speed range of N_(targetMin) to N_(targetMax) for homogenizing engine temperatures.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the time period (t_(motoring)) is calculated automatically during a start of the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller modulates a duty cycle of the starter valve via pulse width modulation.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the starter valve controls the flow of an air supply into the starter of the gas turbine engine.

According to an embodiment, a method for providing a start sequence of a gas turbine engine includes determining a heat state of the gas turbine engine via an engine thermal model. The method includes storing the heat state of the gas turbine engine at shutdown. The method further includes recording a time of the engine shutdown and using a risk model to determine a motoring time period t_(motoring) for a start sequence of the gas turbine engine, wherein the risk model uses the recorded time of the engine shutdown and the stored heat state of the gas turbine engine at shut down to determine the motoring time period t_(motoring).

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the gas turbine engine is motored at a predetermined speed range of N_(targetMin) to N_(targetMax) during the motoring time period, which is less than a normal idle start speed N2.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include dynamically varying a position of a starter valve during the motoring time period in order to motor the gas turbine engine at the predetermined speed range of N_(targetMin) to N_(targetMax).

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the risk model is located on an electronic control of a system programmed to automatically implement the motoring time period.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the heat state is determined by a first thermal model and a second thermal model each being resident upon the electronic engine control system.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include providing a time of an engine start request and an inlet air temperature to the risk model when the risk model determines the motoring time period.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the motoring time period determines the duration of a modified start sequence of the gas turbine engine and the modified start sequence requires motoring of the gas turbine engine at a predetermined speed N_(target) during the motoring time period without introduction of fuel and an ignition source to the gas turbine engine, where the predetermined speed N_(target) is less than a normal idle start speed N2.

A technical effect of the apparatus, systems and methods is achieved by using a start sequence for a gas turbine engine as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic illustration of a starting system for a gas turbine engine in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a starting system for a gas turbine engine in accordance with another embodiment of the disclosure;

FIG. 4 is a block diagram of a system for bowed rotor start mitigation in accordance with an embodiment of the disclosure;

FIG. 5 is a flow chart illustrating a method of bowed rotor start mitigation of a gas turbine engine in accordance with an embodiment of the disclosure;

FIG. 6 is a graph illustrating a bowed rotor risk score with respect to time in accordance with an embodiment of the disclosure;

FIG. 7 is a graph illustrating a normal or cooled engine start versus a modified engine start in accordance with an embodiment of the disclosure;

FIG. 8 is a graph illustrating examples of various vibration level profiles of an engine in accordance with an embodiment of the disclosure;

FIG. 9 is a schematic illustration of a high spool gas path with a straddle-mounted spool in accordance with an embodiment of the disclosure;

FIG. 10 is a schematic illustration of a high spool gas path with an overhung spool in accordance with an embodiment of the disclosure;

FIG. 11 is a graph illustrating commanded starter valve opening with respect to time in accordance with an embodiment of the disclosure; and

FIG. 12 is a graph illustrating a target rotor speed profile of a dry motoring profile and an actual rotor speed versus time in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowed rotor start mitigation system in a gas turbine engine. Embodiments can include using a starter valve to control a rotor speed of a starting spool of the gas turbine engine to mitigate a bowed rotor condition using a dry motoring process. During dry motoring, the starter valve can be actively adjusted to deliver air pressure from an air supply to an engine starting system that controls starting rotor speed. Dry motoring may be performed by running an engine starting system at a lower speed with a longer duration than typically used for engine starting while dynamically adjusting the starter valve to maintain the rotor speed and/or follow a dry motoring profile. Some embodiments increase the rotor speed of the starting spool to approach a critical rotor speed gradually and as thermal distortion is decreased they then accelerate beyond the critical rotor speed to complete the engine starting process. The critical rotor speed refers to a major resonance speed where, if the temperatures are unhomogenized, the combination of a bowed rotor and similarly bowed casing and the resonance would lead to high amplitude oscillation in the rotor and high rubbing of blade tips on one side of the rotor, especially in the high pressure compressor if the rotor is straddle-mounted.

A dry motoring profile for dry motoring can be selected based on various parameters, such as a modeled temperature value of the gas turbine engine used to estimate heat stored in the engine core when a start sequence is initiated and identify a risk of a bowed rotor. The modeled temperature value alone or in combination with other values (e.g., measured temperatures) can be used to calculate a bowed rotor risk parameter. For example, the modeled temperature can be adjusted relative to an ambient temperature when calculating the bowed rotor risk parameter. The bowed rotor risk parameter may be used to take a control action to mitigate the risk of starting the gas turbine engine with a bowed rotor. The control action can include dry motoring consistent with the dry motoring profile. In some embodiments, a targeted rotor speed profile of the dry motoring profile can be adjusted as dry motoring is performed. As one example, if excessive vibration is detected as the rotor speed rises and approaches but remains well below the critical rotor speed, then the rate of rotor speed increases scheduled in the dry motoring profile can be reduced (i.e., a shallower slope) to extend the dry motoring time. Similarly, if vibration levels are observed below an expected minimum vibration level as the rotor speed increases, the dry motoring profile can be adjusted to a higher rate of rotor speed increases to reduce the dry motoring time.

A full authority digital engine control (FADEC) system or other system may send a message to the cockpit to inform the crew of an extended time start time due to bowed rotor mitigation actions prior to completing an engine start sequence. If the engine is in a ground test or in a test stand, a message can be sent to the test stand or cockpit based on the control-calculated risk of a bowed rotor. A test stand crew can be alerted regarding a requirement to keep the starting spool of the engine to a speed below the known resonance speed of the rotor in order to homogenize the temperature of the rotor and the casings about the rotor which also are distorted by temperature non-uniformity.

Monitoring of vibration signatures during the engine starting sequence can also or separately be used to assess the risk that a bowed rotor start has occurred due to some system malfunction and then direct maintenance, for instance, in the case of suspected outer air seal rub especially in the high compressor. Vibration data for the engine can also be monitored after bowed rotor mitigation is performed during an engine start sequence to confirm success of bowed rotor mitigation. If bowed rotor mitigation is unsuccessful or determined to be incomplete by the FADEC, resulting metrics (e.g., time, date, global positioning satellite (GPS) coordinates, vibration level vs. time, etc.) of the attempted bowed rotor mitigation can be recorded and/or transmitted to direct maintenance

Referring now to FIG. 1, a schematic illustration of a gas turbine engine 10 is provided. The gas turbine engine 10 has among other components a fan through which ambient air is propelled into the engine housing, a compressor for pressurizing the air received from the fan and a combustor wherein the compressed air is mixed with fuel and ignited for generating combustion gases. The gas turbine engine 10 further comprises a turbine section for extracting energy from the combustion gases. Fuel is injected into the combustor of the gas turbine engine 10 for mixing with the compressed air from the compressor and ignition of the resultant mixture. The fan, compressor, combustor, and turbine are typically all concentric about a central longitudinal axis of the gas turbine engine 10. Thus, thermal deflection of the components of the gas turbine engine 10 may create the aforementioned bowing or “bowed rotor” condition along the common central longitudinal axis of the gas turbine engine 10 and thus it is desirable to clear or remove the bowed condition prior to the starting or restarting of the gas turbine engine 10.

FIG. 1 schematically illustrates a gas turbine engine 10 that can be used to power an aircraft, for example. The gas turbine engine 10 is disclosed herein as a multi-spool turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. The fan section 22 drives air along a bypass flowpath while the compressor section 24 drives air along a core flowpath for compression and communication into the combustor section 26 then expansion through the turbine section 28. Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment with two turbines and is sometimes referred to as a two spool engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. In both of these architectures the starting spool is that spool that is located around the combustor, meaning the compressor part of the starting spool is flowing directly into the combustor and the combustor flows directly into the turbine section.

The engine 10 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 that interconnects a fan 42, a low pressure compressor 44 and a low pressure turbine 46. The inner shaft 40 is connected to the fan 42 through a geared architecture 48 to drive the fan 42 at a lower speed than the low speed spool 30 in the example of FIG. 1. The high speed spool 32 includes an outer shaft 50 that interconnects a high pressure compressor 52 and high pressure turbine 54. The high speed spool 32 is also referred to as a starting spool, as an engine starting system drives rotation of the high speed spool 32. A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. A mid-turbine frame 57 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 57 further supports bearing systems 38 in the turbine section 28. The inner shaft 40 and the outer shaft 50 are concentric and rotate via bearing systems 38 about the engine central longitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 then the high pressure compressor 52, mixed and burned with fuel in the combustor 56, then expanded over the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 57 includes airfoils 59 which are in the core airflow path. The turbines 46, 54 rotationally drive the respective low speed spool 30 and high speed spool 32 in response to the expansion.

A number of stations for temperature and pressure measurement/computation are defined with respect to the gas turbine engine 10 according to conventional nomenclature. Station 2 is at an inlet of low pressure compressor 44 having a temperature T2 and a pressure P2. Station 2.5 is at an exit of the low pressure compressor 44 having a temperature T2.5 and a pressure P2.5. Station 3 is at an inlet of the combustor 56 having a temperature T3 and a pressure P3. Station 4 is at an exit of the combustor 56 having a temperature T4 and a pressure P4. Station 4.5 is at an exit of the high pressure turbine 54 having a temperature T4.5 and a pressure P4.5. Station 5 is at an exit of the low pressure turbine 46 having a temperature T5 and a pressure P5. Temperatures in embodiments may be measured and/or modeled at one or more stations 2-5. Measured and/or modeled temperatures can be normalized to account for hot day/cold day differences. For instance, measured temperature T2 can be used as an ambient temperature and a modeled temperature (e.g., T3) can be normalized by subtracting measured temperature T2.

Although FIG. 1 depicts one example configuration, it will be understood that embodiments as described herein can cover a wide range of configurations. For example, embodiments may be implemented in a configuration that is described as a “straddle-mounted” spool 32A of FIG. 9. This configuration places two bearing compartments 37A and 39A (which may include a ball bearing and a roller bearing respectively), outside of the plane of most of the compressor disks of high pressure compressor 52A and at outside at least one of the turbine disks of high pressure turbine 54A. In contrast with a straddle-mounted spool arrangement, other embodiments may be implemented using an over-hung mounted spool 32B as depicted in FIG. 10. In over-hung mounted spool 32B, a bearing compartment 37B is located forward of the first turbine disk of high pressure turbine 54B such that the high pressure turbine 54B is overhung, and it is physically located aft of its main supporting structure. The use of straddle-mounted spools has advantages and disadvantages in the design of a gas turbine, but one characteristic of the straddle-mounted design is that the span between the bearing compartments 37A and 39A is long, making the amplitude of the high spot of a bowed rotor greater and the resonance speed that cannot be transited prior to temperature homogenization is lower. For any thrust rating, the straddle mounted arrangement, such as straddle-mounted spool 32A, gives Lsupport/Dhpt values that are higher, and the overhung mounted arrangement, such as overhung spool 32B, can be as much as 60% of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance between bearings (e.g., between bearing compartments 37A and 39A or between bearing compartments 37B and 39B), and Dhpt is the diameter of the last blade of the high pressure turbine (e.g., high pressure turbine 54A or high pressure turbine 54B). As one example, a straddle-mounted engine starting spool, such as straddle-mounted spool 32A, with a roller bearing at bearing compartment 39A located aft of the high pressure turbine 54A may be more vulnerable to bowed rotor problems since the Lsupport/Dhpt ranges from 1.9 to 5.6. FIGS. 9 and 10 also illustrate a starter 120 interfacing via a tower shaft 55 with the straddle-mounted spool 32A proximate high compressor 52A and interfacing via tower shaft 55 with the overhung mounted spool 32B proximate high compressor 52B as part of a starting system.

Turning now to FIG. 2, a schematic of a starting system 100 for the gas turbine engine 10 of FIG. 1 is depicted according to an embodiment. The starting system 100 is also referred to generally as a gas turbine engine system. In the example of FIG. 2, the starting system 100 includes a controller 102 which may be an electronic engine control of an electronic engine control system, such as a dual-channel FADEC, and/or engine health monitoring unit. In an embodiment, the controller 102 may include memory to store instructions that are executed by one or more processors. The executable instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with a controlling and/or monitoring operation of the engine 10 of FIG. 1. The one or more processors can be any type of central processing unit (CPU), including a general purpose processor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Also, in embodiments, the memory may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and control algorithms in a non-transitory form.

The starting system 100 can also include a data storage unit (DSU) 104 that retains data between shutdowns of the gas turbine engine 10 of FIG. 1. The DSU 104 includes non-volatile memory and retains data between cycling of power to the controller 102 and DSU 104. A communication link 106 can include an aircraft and/or test stand communication bus to interface with aircraft controls, e.g., a cockpit, various onboard computer systems, and/or a test stand.

A motoring system 108 is operable to drive rotation of a starting spool (e.g., high speed spool 32) of the gas turbine engine 10 of FIG. 1. Either or both channels of controller 102 can alternate on and off commands to an electromechanical device 110 coupled to a discrete starter valve 116A to achieve a partially open position of the discrete starter valve 116A to control a flow from a starter air supply 114 (also referred to as air supply 114) through a transfer duct 118 to an air turbine starter 120 (also referred to as starter 120 or pneumatic starter motor 120) to drive rotation of a starting spool of the gas turbine engine 10 below an engine idle speed. The air supply 114 (also referred to as starter air supply 114) can be provided by any known source of compressed air, such as an auxiliary power unit or ground cart.

The controller 102 can monitor a speed sensor, such as speed pickup 122 that may sense the speed of the engine rotor through its connection to a gearbox 124 which is in turn connected to the high speed spool 32 via tower shaft 55 (e.g., rotational speed of high speed spool 32) or any other such sensor for detecting or determining the speed of the gas turbine engine 10 of FIG. 1. The starter 120 may be coupled to the gearbox 124 of the gas turbine engine 10 of FIG. 1 directly or through a transmission such as a clutch system (not depicted). The controller 102 can establish a control loop with respect to rotor speed to adjust positioning of the discrete starter valve 116A.

The discrete starter valve 116A is an embodiment of a starter valve that is designed as an on/off valve which is typically commanded to either fully opened or fully closed. However, there is a time lag to achieve the fully open position and the fully closed position. By selectively alternating an on-command time with an off-command time through the electromechanical device 110, intermediate positioning states (i.e., partially opened/closed) can be achieved. The controller 102 can modulate the on and off commands (e.g., as a duty cycle using pulse width modulation) to the electromechanical device 110 to further open the discrete starter valve 116A and increase a rotational speed of the starting spool of the gas turbine engine 10 of FIG. 1. In an embodiment, the electromechanical device 110 has a cycle time defined between an off-command to an on-command to the off-command that is at least half of a movement time for the discrete starter valve 116A to transition from fully closed to fully open. Pneumatic lines 112A and 112B or a mechanical linkage (not depicted) can be used to drive the discrete starter valve 116A between the open position and the closed position. The electromechanical device 110 can be a solenoid that positions the discrete starter valve 116A based on intermittently supplied electric power as commanded by the controller 102. In an alternate embodiment, the electromechanical device 110 is an electric valve controlling muscle air to adjust the position of the discrete starter valve 116A as commanded by the controller 102.

In the example of FIG. 2, the engine also includes a vibration monitoring system 126. The vibration monitoring system 126 includes at least one vibration pickup 128, e.g., an accelerometer, operable to monitor vibration of the gas turbine engine 10 of FIG. 1. Vibration signal processing 130 can be performed locally with respect to the vibration pickup 128, within the controller 102, or through a separate vibration processing system, which may be part of an engine health monitoring system to acquire vibration data 132. Alternatively, the vibration monitoring system 126 can be omitted in some embodiments.

Similar to FIG. 2, FIG. 3 is a schematic illustration of a starting system 100A for the gas turbine engine 10 of FIG. 1 in accordance with another embodiment. The starting system 100A includes controller 102 that controls motoring system 108A, as an alternate embodiment of the motoring system 108 of FIG. 2. Rather than using an electromechanical device 110 coupled to a discrete starter valve 116A to achieve a partially open position of the discrete starter valve 116A of FIG. 2, the motoring system 108A of FIG. 3 uses a variable position starter valve 116B. In FIG. 3, either or both channels of controller 102 can output a valve control signal 150 operable to dynamically adjust a valve angle of the variable position starter valve 116A that selectively allows a portion of the air supply 114 to pass through the variable position starter valve 116B and transfer duct 118 to air turbine starter 120. The variable position starter valve 116B is a continuous/infinitely adjustable valve that can hold a commanded valve angle, which may be expressed in terms of a percentage open/closed and/or an angular value (e.g., degrees or radians). Performance parameters of the variable position starter valve 116B can be selected to meet dynamic response requirements of the starting system 100A. For example, in some embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 40 seconds. In other embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 30 seconds. In further embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 20 seconds.

The controller 102 can monitor a valve angle of the variable position starter valve 116B using valve angle feedback signals 152 provided to both channels of controller 102. As one example, in an active/standby configuration, both channels of the controller 102 can use the valve angle feedback signals 152 to track a current valve angle, while only one channel designated as an active channel outputs valve control signal 150. Upon a failure of the active channel, the standby channel of controller 102 can take over as the active channel to output valve control signal 150. In an alternate embodiment, both channels of controller 102 output all or a portion of a valve angle command simultaneously on the valve control signals 150. The controller 102 can establish an outer control loop with respect to rotor speed and an inner control loop with respect to the valve angle of the variable position starter valve 116B.

As in the example of FIG. 2, the starting system 100A of FIG. 3 also includes vibration monitoring system 126. The vibration monitoring system 126 includes at least one vibration pickup 128, e.g., an accelerometer, operable to monitor vibration of the gas turbine engine 10 of FIG. 1. Vibration signal processing 130 can be performed locally with respect to the vibration pickup 128, within the controller 102, or through a separate vibration processing system, which may be part of an engine health monitoring system to acquire vibration data 132. Alternatively, the vibration monitoring system 126 can be omitted in some embodiments.

FIG. 4 is a block diagram of a system 200 for bowed rotor start mitigation that may control the discrete starter valve 116A of FIG. 2 or the variable position starter valve 116B of FIG. 3 via control signals 210 in accordance with an embodiment. The system 200 may also be referred to as a bowed rotor start mitigation system. In the example of FIG. 4, the system 200 includes an onboard model 202 operable to produce a compressor exit temperature T₃ and a compressor inlet flow W₂₅ for use by a core temperature model 204. The onboard model 202 is configured to synthesize or predict major temperatures and pressures throughout the gas turbine engine 10 of FIG. 1 beyond those sensed by sensors positioned about the gas turbine engine 10. The onboard model 202 and core temperature model 204 are examples of a first thermal model and a second thermal model that may be separately implemented or combined as part of controller 102 and resident upon an electronic engine control system of the gas turbine engine 10 of FIG. 1.

Engine parameter synthesis is performed by the onboard model 202, and the engine parameter synthesis may be performed using the technologies described in U.S. Patent Publication No. 2011/0077783, the entire contents of which are incorporated herein by reference thereto. Of the many parameters synthesized by onboard model 202 at least two are outputted to the core temperature model 204, T₃, which is the compressor exit gas temperature of the engine 10 and W₂₅, which is the air flow through the compressor. Each of these values are synthesized by onboard model 202 and inputted into the core temperature model 204 that synthesizes or provides a heat state (T_(core)) of the gas turbine engine 10. T_(core) can be determined by a first order lag or function of T₃ and a numerical value X (e.g., f(T₃, X)), wherein X is a value determined from a lookup table stored in memory of controller 102. Accordingly, X is dependent upon the synthesized value of W₂₅. In other words, W₂₅ when compared to a lookup table of the core temperature model 204 will determine a value X to be used in determining the heat state or T_(core) of the engine 10. In one embodiment, the higher the value of W₂₅ or the higher the flow rate through the compressor the lower the value of X.

The heat state of the engine 10 during use or T_(core) is determined or synthesized by the core temperature model 204 as the engine 10 is being run. In addition, T₃ and W₂₅ are determined or synthesized by the onboard model 202 and/or the controller 102 as the engine 10 is being operated.

At engine shutdown, the current or most recently determined heat state of the engine or T_(core shutdown) of the engine 10 is recorded into DSU 104, and the time of the engine shutdown t_(shutdown) is recorded into the DSU 104. Time values and other parameters may be received on communication link 106. As long as electrical power is present for the controller 102 and DSU 104, additional values of temperature data may be monitored for comparison with modeled temperature data to validate one or more temperature models (e.g., onboard model 202 and/or core temperature model 204) of the gas turbine engine 10.

During an engine start sequence or restart sequence, a bowed rotor start risk model 206 (also referred to as risk model 206) of the controller 102 is provided with the data stored in the DSU 104, namely T_(core shutdown) and the time of the engine shutdown t_(shutdown). In addition, the bowed rotor start risk model 206 is also provided with the time of engine start t_(start) and the ambient temperature of the air provided to the inlet of the engine 10 T_(inlet) or T₂. T₂ is a sensed value as opposed to the synthesized value of T₃.

The bowed rotor start risk model 206 maps core temperature model data with time data and ambient temperature data to establish a motoring time t_(motoring) as an estimated period of motoring to mitigate a bowed rotor of the gas turbine engine 10. The motoring time t_(motoring) is indicative of a bowed rotor risk parameter computed by the bowed rotor start risk model 206. For example, a higher risk of a bowed rotor may result in a longer duration of dry motoring to reduce a temperature gradient prior to starting the gas turbine engine 10 of FIG. 1. As will be discussed herein and in one embodiment, an engine start sequence may automatically include a modified start sequence; however, the duration of the modified start sequence prior to a normal start sequence will vary based upon the time period t_(motoring) that is calculated by the bowed rotor start risk model 206. The motoring time t_(motoring) for predetermined target speed N_(target) of the engine 10 is calculated as a function of T_(core shutdown), t_(shutdown), t_(start) and T₂, (e.g., f (T_(core shutdown), t_(shutdown), t_(start) and T₂), while a target speed N_(target) is a predetermined speed that can be fixed or vary within a predetermined speed range of N_(targetMin) to N_(targetMax). In other words, the target speed N_(target) may be the same regardless of the calculated time period t_(motoring) or may vary within the predetermined speed range of N_(targetMin) to N_(targetMax). The target speed N_(target) may also be referred to as a dry motoring mode speed.

Based upon these values (T_(core shutdown), t_(shutdown), t_(start) and T₂) the motoring time t_(motoring) at a predetermined target speed N_(target) for the modified start sequence of the engine 10 is determined by the bowed rotor start risk model 206. Based upon the calculated time period t_(motoring) which is calculated as a time to run the engine 10 at a predetermined target speed N_(target) in order to clear a “bowed condition”. In accordance with an embodiment of the disclosure, the controller 102 can run through a modified start sequence upon a start command given to the engine 10 by an operator of the engine 10 such as a pilot of an airplane the engine is used with. It is understood that the motoring time t_(motoring) of the modified start sequence may be in a range of 0 seconds to minutes, which depends on the values of T_(core shutdown), t_(shutdown), t_(start) and T₂.

In an alternate embodiment, the modified start sequence may only be run when the bowed rotor start risk model 206 has determined that the motoring time t_(motoring) is greater than zero seconds upon receipt of a start command given to the engine 10. In this embodiment and if the bowed rotor start risk model 206 has determined that t_(motoring) is not greater than zero seconds, a normal start sequence will be initiated upon receipt of a start command to the engine 10.

Accordingly and during an engine command start, the bowed rotor start risk model 206 of the system 200 may be referenced wherein the bowed rotor start risk model 206 correlates the elapsed time since the last engine shutdown time and the shutdown heat state of the engine 10 as well as the current start time t_(start) and the inlet air temperature T₂ in order to determine the duration of the modified start sequence wherein motoring of the engine 10 at a reduced speed N_(target) without fuel and ignition is required. As used herein, motoring of the engine 10 in a modified start sequence refers to the turning of a starting spool by the starter 120 at a reduced speed N_(target) without introduction of fuel and an ignition source in order to cool the engine 10 to a point wherein a normal start sequence can be implemented without starting the engine 10 in a bowed rotor state. In other words, cool or ambient air is drawn into the engine 10 while motoring the engine 10 at a reduced speed in order to clear the “bowed rotor” condition, which is referred to as a dry motoring mode.

The bowed rotor start risk model 206 can output the motoring time t_(motoring) to a motoring controller 208. The motoring controller 208 uses a dynamic control calculation in order to determine a required valve position of the starter valve 116A, 116B used to supply an air supply or starter air supply 114 to the engine 10 in order to limit the motoring speed of the engine 10 to the target speed N_(target) due to the position of the starter valve 116A, 116B. The required valve position of the starter valve 116A, 116B can be determined based upon an air supply pressure as well as other factors including but not limited to ambient air temperature, parasitic drag on the engine 10 from a variety of engine driven components such as electric generators and hydraulic pumps, and other variables such that the motoring controller 208 closes the loop for an engine motoring speed target N_(target) for the required amount of time based on the output of the bowed rotor start risk model 206. In one embodiment, the dynamic control of the valve position (e.g., open state of the valve (e.g., fully open, ½ open, ¼ open, etc.) in order to limit the motoring speed of the engine 10) is controlled via duty cycle control (on/off timing using pulse width modulation) of electromechanical device 110 for discrete starter valve 116A.

When the variable position starter valve 116B of FIG. 3 is used, a valve angle 207 can be provided to motoring control 208 based on the valve angle feedback 152 of FIG. 3. A rotor speed N2 (i.e., speed of high speed spool 32) can be provided to the motoring controller 208 and a mitigation monitor 214, where motoring controller 208 and a mitigation monitor 214 may be part of controller 102. Vibration data 132 can also be provided to mitigation monitor 214.

The risk model 206 can determine a bowed rotor risk parameter that is based on the heat stored (T_(core)) using a mapping function or lookup table. When not implemented as a fixed rotor speed, the bowed rotor risk parameter can have an associated dry motoring profile defining a target rotor speed profile over an anticipated amount of time for the motoring controller 208 to send control signals 210, such as valve control signals 150 for controlling variable position starter valve 116B of FIG. 3.

The bowed rotor risk parameter may be quantified according to a profile curve 402 selected from a family of curves 404 that align with observed aircraft/engine conditions that impact turbine bore temperature and the resulting bowed rotor risk as depicted in the example graph 400 of FIG. 5.

In some embodiments, an anticipated amount of dry motoring time can be used to determine a target rotor speed profile in a dry motoring profile for the currently observed conditions. As one example, one or more baseline characteristic curves for the target rotor speed profile can be defined in tables or according to functions that may be rescaled to align with the observed conditions. An example of a target rotor speed profile 1002 is depicted in graph 1000 of FIG. 12 that includes a steep initial transition portion 1004, followed by a gradually increasing portion 1006, and a late acceleration portion 1008 that increases rotor speed above a critical rotor speed, through a fuel-on speed and an engine idle speed. The target rotor speed profile 1002 can be rescaled with respect to time and/or select portions (e.g., portions 1004, 1006, 1008) of the target rotor speed profile 1002 can be individually or collectively rescaled (e.g., slope changes) with respect to time to extend or reduce the total motoring time. The target rotor speed profile 1002 may include all positive slope values such that the actual rotor speed 1010 is driven to essentially increase continuously while bowed rotor start mitigation is active. While the example of FIG. 12 depicts one example of the target rotor speed profile 1002 that can be defined in a dry motoring profile, it will be understood that many variations are possible in embodiments.

An example of the effects of bowed rotor mitigation are illustrated in graph 420 of FIG. 7 that depicts a normal or cooled engine start (line 432) versus a bowed rotor or mitigated engine start (line 434) in accordance with one non-limiting embodiment of the disclosure. At point 436, a pilot or operator of the engine 10 sets or initiates a start command of the engine. At point 438 and after the start command is initiated, the controller 102, based upon the risk model 206, requires the engine to motor at a pre-determined speed (N_(pre-determined) or N_(target)), which is less than a normal idle start speed N2 for a time (t_(determined)). The pre-determined speed (N_(pre-determined) or N_(target)) can be defined within a predetermined speed range N_(targetMin) to N_(targetMax) that is used regardless of the calculated time period t_(motoring) for homogenizing engine temperatures. The time period t_(determined) is based upon the output of the risk model 206. The determined speed (N_(pre-determined) or N_(target)) is achieved by controlling the operational position of starter valve 116A, 116B. Thereafter and at point 440 when the required motoring time (determined from the risk model 206) has been achieved, such that the “bowed condition” has been cleared a normal start sequence with a normal speed N2 is initiated. Subsequently and at point 442, the idle speed N2 has been achieved. This modified sequence is illustrated in one non-limiting manner by the dashed line 434 of the graph 420 of FIG. 7. It is, of course, understood that (t_(determined)) may vary depending upon the outputs of the risk model 206, while N_(pre-determined) or N_(target) is a known value. Of course, in alternative embodiments, the risk model 206 may be configured to provide the speed of the engine 10 during a modified start sequence. Still further and as mentioned above, the starter valve may be dynamically varied based upon the outputs of the risk model 206 as well as the pressure of the air supply 114 in order to limit the motoring speed of the engine 10 to that of N_(pre-determined) or N_(target) during the clearing of a bowed rotor condition. Line 432 illustrates a normal start sequence wherein the time t_(determined) is zero for a modified start as determined by the risk model 206.

The example of FIG. 11 illustrates how a valve angle command 902 can be adjusted between 0 to 100% of a commanded starter valve opening to generate the actual rotor speed 1010 of FIG. 12. As the actual rotor speed 1010 tracks to the steep initial transition portion 1004 of the target rotor speed profile 1002, the valve angle command 902 transitions through points “a” and “b” to fully open the variable position starter valve 116B. As the slope of the target rotor speed profile 1002 is reduced in the gradually increasing portion 1006, the valve angle command 902 is reduced between points “b” and “c” to prevent the actual rotor speed 1010 from overshooting the target rotor speed profile 1002. In some embodiments, decisions to increase or decrease the commanded starter valve opening is based on monitoring a rate of change of the actual rotor speed 1010 and projecting whether the actual rotor speed 1010 will align with the target rotor speed profile 1002 at a future time. If it is determined that the actual rotor speed 1010 will not align with the target rotor speed profile 1002 at a future time, then the valve angle of the variable position starter valve 116B is adjusted (e.g., increase or decrease the valve angle command 902) at a corresponding time. In the example of FIGS. 11 and 12, the valve angle command 902 oscillates with a gradually reduced amplitude between points “c”, “d”, and “e” as the actual rotor speed 1010 tracks to the target rotor speed profile 1002 through the gradually increasing portion 1006. As dry motoring continues, the overall homogenization of the engine 10 increases, which allows the actual rotor speed 1010 to safely approach the critical rotor speed without risking damage. The valve angle command transitions from point “e” to point “f” and beyond to further increase the actual rotor speed 1010 in the late acceleration portion 1008 above the critical rotor speed, through a fuel-on speed and an engine idle speed. By continuously increasing the actual rotor speed 1010 during dry motoring, the bowed rotor condition can be reduced faster than holding a constant slower speed.

In summary with reference to FIG. 4, as one example of an aircraft that includes systems as described herein, onboard model 202 and core temperature model 204 may run on controller 102 of the aircraft to track heat stored (T_(ore)) in the turbine at the time of engine shutdown. Modeling of potential heat stored in the system may be performed as a turbine disk metal temperature model in the core temperature model 204. When the aircraft lands, engines typically operate at idle for a cool down period of time, e.g., while taxiing to a final destination. When an engine shutdown is detected, model state data can be logged by the DSU 104 prior to depowering. When the controller 102 powers on at a later time and model state data can be retrieved from the DSU 104, and the bowed rotor start risk model 206 can be updated to account for the elapsed time. When an engine start is requested, a bowed rotor risk can be assessed with respect to the bowed rotor start risk model 206. Extended dry motoring can be performed during an engine start process until the bow risk has sufficiently diminished. Peak vibrations can be checked by the mitigation monitor 214 during the start processes to confirm that bowed rotor mitigation successfully removed the bowed rotor condition.

In reference to FIGS. 4 and 12, the mitigation monitor 214 of FIG. 4 can operate in response to receiving a complete indicator 212 to run a verification of the bowed rotor mitigation. The mitigation monitor 214 can provide mitigation results 216 to the motoring controller 208 and may provide result metrics 218 to other systems, such a maintenance request or indicator. Peak vibrations can be checked by the mitigation monitor 214 during the start processes to confirm that bowed rotor mitigation successfully removed the bowed rotor condition. The mitigation monitor 214 may also run while dry motoring is active to determine whether adjustments to the dry motoring profile are needed. For example, if a greater amount of vibration is detected than was expected, the mitigation monitor 214 can request that the motoring controller 208 reduce a slope of the target rotor speed profile 1002 of FIG. 12 to extend the dry motoring time before driving the actual rotor speed 1010 of FIG. 12 up to the critical rotor speed. Similarly, if the magnitude of vibration observed by the mitigation monitor 214 is less than expected, the mitigation monitor 214 can request that the motoring controller 208 increase a slope of the target rotor speed profile 1002 of FIG. 12 to reduce the dry motoring time before driving the actual rotor speed 1010 of FIG. 12 up to the critical rotor speed.

FIG. 5 is a flow chart illustrating a method 300 of bowed rotor start mitigation of the gas turbine engine 10 in accordance with an embodiment. The method 300 of FIG. 5 is described in reference to FIGS. 1-12 and may be performed with an alternate order and include additional steps. Before initiating bowed rotor start mitigation, a bowed rotor determination step can be performed to estimate a need for bowed rotor start mitigation. Examples include the use of models and/or stored/observed engine/aircraft state data, such as data received from DSU 104, communication link 106, and/or reading data from one or more temperature sensors of the gas turbine engine 10.

At block 302, the controller 102 determines a heat state (T_(core)) of the gas turbine engine 10 via an engine thermal model (e.g., onboard model 202 and core temperature model 204 of FIG. 4). At block 304, the controller 102 stores the heat state of the gas turbine engine at shutdown into DSU 104. At block 306, the controller 102 records a time of the engine shutdown in DSU 104.

At block 308, the controller 102 uses risk model 206 to determine a motoring time period t_(motoring) for a start sequence of the gas turbine engine 10, where the risk model 206 uses the recorded time of the engine shutdown and the stored heat state of the gas turbine engine 10 at shut down to determine the motoring time period t_(motoring). The gas turbine engine 10 is motored at a predetermined speed range of N_(targetMin) to N_(targetMax) during the motoring time period, which is less than a normal idle start speed N2. The controller 102 can dynamically vary a position of starter valve 116A, 116B during the motoring time period in order to motor the gas turbine engine 10 at the predetermined speed range of N_(targetMin) to N_(targetMax). The predetermined speed range of N_(targetMin) to N_(targetMax) may be tightly controlled to a substantially constant rotor speed or cover a wider operating range according to a dry motoring profile.

As one example with respect to FIGS. 3 and 12, the variable position starter valve 116B can be initially set to a valve angle of greater than 50% open when bowed rotor start mitigation is active. The controller 102 can monitor a rate of change of the actual rotor speed 1010, project whether the actual rotor speed 1010 will align with the target rotor speed profile 1002 at a future time based on the rate of change of the actual rotor speed 1010, and adjust a valve angle of the variable position starter valve 116B based on determining that the actual rotor speed 1010 will not align with the target rotor speed profile 1002 at a future time.

Further dynamic updates at runtime can include adjusting a slope of the target rotor speed profile 1002 in the dry motoring profile while the bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine 10 is outside of an expected range. Adjusting the slope of the target rotor speed profile 1002 can include maintaining a positive slope. Vibration levels may also or alternatively be used to check/confirm successful completion of bowed rotor start mitigation prior to starting the gas turbine engine 10. For instance, based on determining that the bowed rotor start mitigation is complete, a vibration level of the gas turbine engine 10 can be monitored while sweeping through a range of rotor speeds including the critical rotor speed.

In further reference to FIG. 4, the mitigation monitor 214 of FIG. 4 may receive a complete indicator 212 from the motoring controller 208 when the motoring controller 208 has completed dry motoring, for instance, if the motoring time has elapsed. If the mitigation monitor 214 determines that the bowed rotor condition still exists based on vibration data 132 collected, the motoring controller 208 may restart dry motoring, or a maintenance request or indicator can be triggered along with providing result metrics 218 for further analysis. Metrics of attempted bowed rotor mitigation can be recorded in the DSU 104 based on determining that the attempted bowed rotor mitigation was unsuccessful or incomplete.

Referring now to FIG. 8, a graph 500 illustrating examples of various vibration level profiles 502 of an engine, such as gas turbine engine 10 of FIG. 1 is depicted. The vibration level profiles 502 represent a variety of possible vibration levels observed before and/or after performing bowed rotor mitigation. Critical rotor speed 510 is the speed at which a vibration peak is expected due to amplification effects of a bowed rotor condition along with other contributions to vibration level generally. A peak vibration 504 at critical rotor speed 510 may be used to trigger different events. For example, if the peak vibration 504 at critical rotor speed 510 is below a maintenance action threshold 506, then no further actions may be needed. If the peak vibration 504 at critical rotor speed 510 is above a damage risk threshold 508, then an urgent maintenance action may be requested such as an engine check. If the peak vibration 504 at critical rotor speed 510 is between the maintenance action threshold 506 and the damage risk threshold 508, then further bowed rotor mitigation actions may be requested, such as extending/restarting dry motoring. In one embodiment, a maintenance request is triggered based on the actual vibration level exceeding maintenance action threshold 506 after completing an attempt of bowed rotor mitigation.

The lowest rotor vibration vs. speed in FIG. 8 (vibration profile 502D) is for a fully homogenized rotor, where mitigation is not necessary (engine parked all night long, for example). The next higher curve shows a mildly bowed rotor and so on. The maintenance action threshold 506 is a threshold for setting a maintenance flag such as requiring a troubleshooting routine of one or more system elements. The damage risk threshold 508 may be a threshold to trigger a more urgent maintenance requirement up to and including an engine check. As dry motoring is performed in embodiments, the gas turbine engine 10 may shift between vibration profiles. For instance, when a bow rotor condition is present, the gas turbine engine 10 may experience vibration levels according to vibration profile 502A, if mitigation is not performed. As dry motoring is run, the gas turbine engine 10 may have a vibration profile that is gradually reduced from vibration profile 502A to vibration profile 502B and then vibration profile 502C, for example. By checking the current vibration level at a corresponding rotor speed with respect to time, the controller 102 can determine whether adjustments are needed to extend or reduce the slope of the target rotor speed profile 1002 of FIG. 12 depending on an expected rate of bowed rotor reduction. In embodiments, a slope of the target rotor speed profile 1002 in the dry motoring profile 206 can be adjusted and maintains a positive slope while bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine 10 is less than a targeted maximum range 512, which may define a safe level of vibration to ensure that no risk of a maintenance action or damage will likely occur if the actual rotor speed 1010 is increased faster than previously planned.

Accordingly and as mentioned above, it is desirable to detect, prevent and/or clear a “bowed rotor” condition in a gas turbine engine that may occur after the engine has been shut down. As described herein and in one non-limiting embodiment, the controller 102 may be programmed to automatically take the necessary measures in order to provide for a modified start sequence without pilot intervention other than the initial start request. In an exemplary embodiment, the controller 102 and/or DSU 104 comprises a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm and/or algorithms that control the start sequence of the gas turbine engine. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), the controller 102 and/or DSU 104 may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, the controller 102 and/or DSU 104 may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments of the disclosure can be implemented through computer-implemented processes and apparatuses for practicing those processes.

While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A system for starting a gas turbine engine of an aircraft, the system comprising: a pneumatic starter motor; a discrete starter valve switchable between an on-state and an off-state; and a controller operable to perform a starting sequence for the gas turbine engine, the starting sequence comprising alternating on and off commands to an electromechanical device coupled to the discrete starter valve to achieve a partially open position of the discrete starter valve to control a flow from a starter air supply to the pneumatic starter motor to drive rotation of a starting spool of the gas turbine engine below an engine idle speed.
 2. The system as in claim 1, wherein the electromechanical device has a cycle time defined between an off-command to an on-command to the off-command that is at most half of a movement time for the discrete starter valve to transition from fully closed to fully open.
 3. The system as in claim 1, wherein the electromechanical device is a solenoid that positions the discrete starter valve based on intermittently supplied electric power.
 4. The system as in claim 1, wherein the electromechanical device is an electric valve controlling muscle air to adjust the position of the discrete starter valve.
 5. The system as in claim 1, wherein the controller modulates the on and off commands to the electromechanical device to further open the discrete starter valve and increase a rotational speed of the starting spool.
 6. The system as is claim 1, further comprising: an electronic engine control system comprising: a memory for recording a current heat state of the gas turbine engine at shutdown and for recording a shutdown time of the gas turbine engine; and a risk model for determining a time period (t_(motoring)) for motoring the gas turbine engine at a predetermined speed N_(target) wherein the predetermined speed is less than a speed used to start the gas turbine engine and wherein t_(motoring) is a function of the heat state recorded at engine shutdown and an elapsed time of an engine start request relative to the previous shutdown time.
 7. The system as in claim 6, wherein the time period (t_(motoring)) is calculated automatically during a start of the gas turbine engine.
 8. The system as in claim 6, wherein the controller modulates a duty cycle of the discrete starter valve via pulse width modulation.
 9. The system as in claim 6, wherein the predetermined speed N_(target) is within a predetermined speed range N_(targetMin) to N_(targetMax) that is used regardless of the calculated time period t_(motoring).
 10. A gas turbine engine and a system for controlling a start sequence of the gas turbine engine, comprising: an electronic engine control system; a thermal model resident upon the electronic engine control system and configured to synthesize a heat state of the gas turbine engine; a memory for recording the current heat state of the gas turbine engine at shutdown and for recording a shutdown time of the gas turbine engine; a model for determining a time period (t_(motoring)) for motoring the gas turbine engine at a predetermined speed N_(target) wherein the predetermined speed is less than a speed used to start the gas turbine engine and wherein t_(motoring) is a function of the heat state recorded at engine shutdown and an elapsed time of an engine start request relative to a previous shutdown time; and a controller for modulating a starter valve of a starter of the gas turbine engine in order to maintain the gas turbine engine within a predetermined speed range of N_(targetMin) to N_(targetMax) for homogenizing engine temperatures.
 11. The gas turbine engine and system of claim 10, wherein the time period (t_(motoring)) is calculated automatically during a start of the gas turbine engine.
 12. The gas turbine engine and system of claim 10, wherein the controller modulates a duty cycle of the starter valve via pulse width modulation.
 13. The gas turbine engine and system of claim 10, wherein the starter valve controls the flow of an air supply into the starter of the gas turbine engine.
 14. A method for providing a start sequence of a gas turbine engine, comprising: determining a heat state of the gas turbine engine via an engine thermal model; storing the heat state of the gas turbine engine at shutdown; recording a time of the engine shutdown; and using a risk model to determine a motoring time period t_(motoring) for a start sequence of the gas turbine engine, wherein the risk model uses the recorded time of the engine shutdown and the stored heat state of the gas turbine engine at shut down to determine the motoring time period t_(motoring).
 15. The method as in claim 14, wherein the gas turbine engine is motored at a predetermined speed range of N_(targetMin) to N_(targetMax) during the motoring time period, which is less than a normal idle start speed N2.
 16. The method as in claim 15, further comprising dynamically varying a position of a starter valve during the motoring time period in order to motor the gas turbine engine at the predetermined speed range of N_(targetMin) to N_(targetMax).
 17. The method as in claim 14, wherein the risk model is located on an electronic control of a system programmed to automatically implement the motoring time period.
 18. The method as in claim 17, wherein the heat state is determined by a first thermal model and a second thermal model each being resident upon the electronic engine control system.
 19. The method as in claim 14, further comprising providing a time of an engine start request and an inlet air temperature to the risk model when the risk model determines the motoring time period.
 20. The method as in claim 14, wherein the motoring time period determines the duration of a modified start sequence of the gas turbine engine and the modified start sequence requires motoring of the gas turbine engine at a predetermined speed N_(target) during the motoring time period without introduction of fuel and an ignition source to the gas turbine engine, wherein the predetermined speed N_(target) is less than a normal idle start speed N2. 